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Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst
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Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst Malinee Kaewpanha a, Guoqing Guan a,b,*, Yufei Ma a, Xiaogang Hao c, Zhonglin Zhang c, Prasert Reubroychareon d, Katsuki Kusakabe e, Abuliti Abudula a,b,* a
Graduate School of Science and Technology, Hirosaki University, 1-Bunkyocho, Hirosaki 036-8560, Japan North Japan Research Institute for Sustainable Energy (NJRISE), Hirosaki University, 2-1-3, Matsubara, Aomori 030-0813, Japan c Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China d Department of Chemical Technology, Chulalongkorn University, Bangkok 10330, Thailand e Department of Nanoscience, Sojo University, 4-22-1 Ikeda, Nishi-ku, Kumamoto 860-0082, Japan b
article info
abstract
Article history:
Catalytic steam reforming of tar derived from Japanese cedar pyrolysis was investigated
Received 2 March 2015
over a prepared biomass char supported molybdenum carbide (Mo2C/BC) catalyst in a
Received in revised form
fixed-bed reactor. Mo2C/BC was prepared by in-situ solid state reaction, and characterized
26 March 2015
by XRD and SEM to study the effects of carburization temperature and Mo loading amount
Accepted 14 April 2015
on Mo2C formation. The results show that Mo2C/BC catalyst was successfully prepared at a
Available online xxx
relatively low carburization temperature of 800 C and the initial Mo loading amount for the preparation of this catalyst should be lower than 30 wt.%. The Mo2C/BC showed a good
Keywords:
catalytic activity for the steam reforming of tar derived from biomass. In the case of
Biomass
Mo2C/BC with a Mo loading of 20 wt.%, the highest H2 yield was obtained, which is about 5
Tar reforming
times higher than that of non-catalytic test. Mo2C/BC could be a promising catalyst in
Molybdenum carbide
steam gasification of biomass to remove tar and produce H2-rich gas.
Biomass char
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Catalytic activity
Introduction Nowadays, energy and environmental issues are two common concerns that should be managed simultaneously. At present, although fossil fuels such as oil, coal and natural gas are the
reserved.
major energy sources for global energy requirement, they are finite and will be depleted eventually, and also the major cause for global warming due to the emission of greenhouse gas during combustion of fossil fuels. According to the two issues stated above, considerable efforts have been made to develop or find the alternative fuels that can solve both the
* Corresponding authors. North Japan Research Institute for Sustainable Energy (NJRISE), Hirosaki University, 2-1-3, Matsubara, Aomori 030-0813, Japan. Tel.: þ81 17 762 7756; fax: þ81 17 735 5411. E-mail addresses: [email protected] (G. Guan), [email protected] (A. Abudula). http://dx.doi.org/10.1016/j.ijhydene.2015.04.068 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068
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mentioned problems. Hydrogen has been proposed as one of the most promising energy carriers in the future due to its cleanliness and high-energy yield. However, approximately 96% of hydrogen is produced from fossil fuels, mainly by steam reforming of natural gas. In such cases, a significant amount of CO2 is released during the hydrogen production [1,2]. Biomass gasification is a promising way to produce hydrogen or synthesis gas, which is a high value gaseous product and can be applied in fuel cells for power generation or production of liquid fuels [3e7]. Biomass is renewable, abundant and carbon neutral. Nevertheless, the major problem of biomass gasification is the formation of tar, which is a complex mixture of aromatics and undesirable products. Tar can remain liquid at room temperature, causing plugging, fouling and corrosion in the equipment downstream [8e12]. Therefore, tar removal step from the syngas is imperative before syngas applications. There are several methods to reduce the tar content in raw fuel gas by physical and chemical processes. Among them, catalytic steam reforming is a very attractive technique for tar removal since it can remove tar effectively and simultaneously convert tar into useful gaseous products (H2 and CO) [13e16]. Many kinds of catalysts have been developed for catalytic conversion of tar. Noble metal catalysts such as Ru, Rh, and Pt showed high catalytic activity in the steam reforming of tar with high sulfur resistance and long term stability [17,18]. However, noble metals are expensive, making them not be used in the conventional process. Ni-based catalysts have been used extensively for biomass tar conversion because of their high tar destruction activity. However, the main limitation of nickel catalysts is the rapid deactivation, caused by carbon formation on the catalyst surface [19e21]. Moreover, other metal catalysts like Co, Fe, Zn, and Cu have also been investigated in steam reforming of tar and showed higher catalytic activity than Ni catalyst in some cases. Although other metal catalysts exhibit a good performance in steam reforming of tar, they are still deactivated easily by sulfur or high heavy tar content [1,22e24]. In recent years, molybdenum carbide (Mo2C) has been reported to have high catalytic activity similar to precious metals in various reactions such as methanol reforming, hydrogenation reactions, water gas shift, and methane reforming [24e28]. Especially, the raw materials for synthesis of Mo2C are abundant and inexpensive. Generally, metal carbide catalysts could be prepared through pyrolysis, self-propagating high temperature synthesis and direct carburization at high temperature up to 2000 C. However, these methods require very specialized high temperature equipment, gadget and facilities [29]. In order to reduce the production cost of such catalysts, several methods have been developed for the preparation of metal carbide at relatively low temperature. For instance, Mordenti et al. [30] have reported that the formation of Mo2C phase at low temperature (1000 C) was obtained by reaction between the transition metal oxide and high specific surface activated carbon. Chen et al. [31] studied the synthesis of Mo2C which is covalently anchored to carbon supports, and found that active catalysts (Mo2C) can be produced by solidestate reaction of original biomass (soybean) and ammonium molybdate at relatively low temperature (800 C). In this work, we tried to use carbon in original biomass as the carbon source
and find a simple preparation method to prepare Mo2C based catalysts. Herein, Mo2C/cedar char catalysts were prepared by in-situ solid state reaction. The obtained catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and BET surface area measurements, and then tested their catalytic activities for the steam reforming of tar derived from biomass pyrolysis. The effects of Mo loading amount and carburization temperature on the formation of molybdenum carbide and their catalytic activities were investigated and discussed.
Experimental Materials and catalyst preparation In this study, cedar wood (with particle size of 1.0e2.8 mm) was used as the carbon sources. It was dried in an oven at 105 C before storage and further use. The water content of the dried cedar wood was 8.0 wt.%. The water-free compositions by weight percentage were C 48.8%, H 6.6%, O 43.0%, N 1.4%, and ash 0.6%. From XRF analysis, the main compositions in ash by weight percentage were CaO 50.68%, SO3 10.47%, SiO2 6.83%, P2O5 6.45%, K2O 4.01%, Fe2O3 0.80% and SrO 0.04%. Ammonium heptamolybdate ((NH4)6Mo7O24$4H2O) was purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan) and used directly. Biomass char supported Mo2C (Mo2C/BC) catalyst was prepared by the following procedures: (1) dried cedar was directly impregnated by wet impregnation method with an aqueous solution of ammonium heptamolybdate of appropriate concentration to load 5-30 wt.% on initial Mo basis; (2) the slurry was dried at 110 C overnight; (3) the solid mixture was carburized in a vertical tube furnace with a 50 cm3/min Ar flow from room temperature (RT) to the desired temperatures and then held for 2 h. The carburization was performed at the temperature range of 600e800 C. Ar was kept purging through the reactor when the reactor was cooled down to room temperature before storage and further use. To evaluate the effect of carburization temperature on the Mo2C formation, ammonium heptamolybdate deposited dried cedar (initial Mo loading amount of 10 wt.%) was carburized at different carburization temperatures of 600, 700 and 800 C, separately. The samples were heated at rapid rate from room temperature to the final carburization temperature and held at that temperature for 2 h. For regeneration of the asprepared Mo2C/BC catalyst, the spent catalyst was recarburized in Ar atmosphere at temperature of 800 and 900 C, separately, using the same procedure as a carburization of Mo2C/BC catalyst.
Characterization of catalyst XRD analysis was conducted using a XRD 610 (Shimadzu, Japan) to determine the crystal structure of the as-prepared catalyst. The morphology of as-prepared catalyst was characterized using a SEM (SU6600, Hitachi) equipped with energy dispersive spectrometer (EDS). Catalyst surface areas and pore volumes were measured using BET sorption isotherm method (Quantachrome NOVA 4200e, USA)
Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068
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Catalytic activity test Catalytic activity in the steam reforming of tar was tested in a down-flow fixed-bed reactor with an internal diameter and a length of 18 and 350 mm, respectively. The experimental setup was described in details in our previous study [32]. 0.6 g of cedar powders and Mo2C/BC catalyst with different amounts were introduced into the reactor and separated with a layer of quartz wool. The height of catalyst bed was kept constant about 8 cm for all experiments. The reactions were carried out at atmospheric pressure. Reaction temperatures were chosen in the range of 550e750 C. Heating rate was fixed at 20 C/min. Water with a flow rate of 0.09 g/min was introduced using a peristaltic pump into a steam generator (250 C) at first, and then was carried to the reactor by argon gas with a flow rate 50 cm3/min. The produced gases were passed through two cooling baths under 0 C and a dry cylinder with CaCl2 particles, and collected using a gas bag. The gas compositions were analyzed using an Agilent 7890A gas chromatograph system. The carbon content in the remained char was estimated by using Vario EL cube elemental analyzer. The heavy tars were collected in the bottle with acetone, and then the water and acetone were evaporated at 100 C for 24 h in an oven, and the remained heavy tars were weighed. For the catalytic system, the gas production yield (Ybio) from the gasification of cedar and the catalytic steam reforming of tar derived from cedar was calculated as follow: Ybio ¼ Ytotal Ycat
(1)
where Ytotal is the gas production yield in the catalytic system (consisting of the gas production yield from the gasification of cedar, the catalytic steam reforming of tar derived from cedar and the gasification of catalyst itself) while Ycat is the gas production yield when catalyst itself was gasified at the same condition.
Results and discussion Characterization of the as-prepared molybdenum carbides Effect of carburization temperature As stated above, Mo2C/BC catalysts were prepared by in-situ solid state reaction from dried cedar and ammonium heptamolybdate. As shown in Fig. 1, it can be seen that at a low carburization temperature, i.e., 600 C, only molybdenum oxide (MoO2) phase was observed. As the temperature was increased to 700 C, it is found that the signal of MoO2 phase decreased and the signal of Mo2C began to appear. When the temperature was increased to 800 C, the XRD pattern shows the peaks of pure Mo2C, indicating that Mo2C was successfully prepared by in-situ solid state reaction of dried cedar and ammonium heptamolybdate. All these results indicate that the minimum carburization temperature for Mo2C formation is 800 C under the conditions selected in this work. According to the XRD results, as the Mo2C formation pathway reported in literatures [33,34], the carburization steps for Mo2C/BC catalyst were illustrated in Scheme 1.
Fig. 1 e XRD patterns of carburized products at different carburization temperatures.
Ammonium heptamolybdate should be decomposed to MoO3 (Mo6þ) at the lower temperature step and then, the generated MoO3 (Mo6þ) is continuously reduced to MoO2 (Mo4þ). Thereafter, MoO2 (Mo4þ) is continuously reduced to metallic molybdenum and further transformed into Mo2C at the high carburization step. Especially, the use of dried biomass instead of pure carbon materials (such as biomass char) as the carbon source in this study could promote the Mo2C formation and reduce the carburization temperature. This is due to the reducing gases such as CO and H2, which are produced during biomass pyrolysis. These can provide reduction atmosphere to reduce the molybdenum oxide, leading to the formation of Mo2C more easily. Furthermore, carbon monoxide and methane are considered as reduction and carburizing gases, respectively, for metal carbide preparation [30]. For comparison, we had ever used cedar char as the carbon source, and found that the carburization temperature should be increased to over 900 C in order to obtain pure Mo2C supported on char (as shown in Fig. 2).
Effect of Mo loading amount In order to investigate the effect of Mo loading amount on the Mo2C formation, dried cedar was impregnated with an aqueous solution of ammonium heptamolybdate with different contents in the range of 5e30 wt.% on Mo basis. The samples were carburized at a temperature of 800 C for 2 h and characterized by XRD. Fig. 3 shows the XRD patterns of carburized products prepared under different Mo loading amounts. It is found that pure Mo2C phase can be obtained at initial Mo loading amount below 30 wt.%, and the signals of Mo2C phase increased with the increase in Mo amount due to the formation of better-crystalline Mo2C particles. However, further increased initial Mo loading amount to 30 wt.%, the XRD pattern showed two different crystalline phases (1) a main Mo2C phase, and (2) traces of MoO2 phase. It is indicated that due to the excess of Mo loading amount in this
Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068
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Scheme 1 e Proposed formation process of biomass char supported Mo2C catalyst by in-situ solid state reaction.
experiment, some Mo could still remain in the oxidation state. pez et al. [34], Similar results were also reported by Guil-Lo who investigated the influence of the Mo-loading on the crystalline Mo-phases during the carburization processes. Their results revealed that at high Mo-loading amount, the carburization of the molybdate precursor was not complete and a higher carburization temperature was required to fulfill Mo-carbide formation. Therefore, based on the XRD results, the initial Mo loading amount for Mo2C/BC catalyst in this study should be lower than 30 wt.% at which well-crystalline Mo2C was generated in this one-step solidesolid carburization process at relatively low carburization temperature of 800 C. The BET surface area and pore volume of the as-prepared Mo2C/BC catalyst with different initial Mo loading amount are given in Table 1. It can be seen that the surface area and pore volume of the samples decreased with increasing of Mo loading amount. However, all catalysts have high surface areas, which
Three main ways which can enhance the biomass gasification rate and increase the hydrogen production yield are always considered. That is, (1) using steam gasification instead of partial oxidation, (2) increasing reaction temperature, and (3) catalyst assistance [35]. In the present study, the catalytic activity tests of the as-prepared Mo2C/BC catalysts toward tar steam reforming were performed in a
Fig. 2 e XRD patterns of carburized products at different carburization temperatures when using cedar char as carbon source.
Fig. 3 e XRD patterns of carburized products at different Mo loading amounts (Carburization reaction temperature ¼ 800 C).
are benefit for the catalyst. Scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) was also used to explore the morphology of the Mo2C/BC catalysts. Fig. 4 shows the SEM images and the EDS mapping image of ammonium heptamolybdate deposited dried cedar (initial Mo loading amount of 20 wt.%) after carburization at 800 C for 2 h. It indicated the existence of Mo with good dispersion on the surface of the as-prepared catalyst.
Catalytic activity of Mo2C/BC catalyst
Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068
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Table 1 e Surface area, pore volume and crystallite size of the as-prepared catalysts with different initial Mo loading amounts. Initial Mo loading amount (wt.%) 5 10 15 20 30 a
Surface area (m2 g1)
Pore volume (cm3 g1)
Crystallite sizea (nm)
422.0 362.3 323.4 317.8 247.1
0.231 0.201 0.198 0.193 0.169
24.2 24.6 25.4 25.7 29.9
Crystallite size of Mo2C is estimated using the Scherrer formula.
fixed-bed reactor with different conditions. In general, when tar cracking is performed in the presence of steam, the general steam reforming reaction can be expressed as following:
Fig. 4 e SEM (a,b) and EDS mapping (c) photographs of Mo2C/cedar char catalyst (Mo loading amount ¼ 20 wt.%; carburization reaction temperature ¼ 800 C).
Cx Hy Oz þ ðx zÞH2 O/xCO þ ½ðx þ y=2 zÞH2
5
(2)
and the produced CO could react further with excess steam and is converted to CO2 and H2 by water-gas shift (WGS) reaction:
CO þ H2O 4 CO2 þ H2
(3)
Fig. 5 shows the yield (Ybio) of each gas, which was calculated by Eq. (1), from catalytic steam gasification of biomass using Mo2C/BC catalysts with different initial Mo loading amounts. Here, the experiments were performed at a reaction temperature of 650 C with a water flow rate of 0.09 g/min at room temperature. Results were compared to those obtained from non-catalytic system. It is found that significant changes in the gas production yields due to the presence of catalyst, especially for H2 and CO2. According to Eqs. (2) and (3), the results can be explained by the fact that biomass tar is decomposed first over the catalyst to gaseous components such as CO and H2 and then, the produced CO reacts further with excess steam by water-gas shift reaction, leading to high H2 and CO2 yields. For different initial Mo loading amounts, the results showed that Ybio increased with increasing of Mo loading amount at the beginning, and decreased slightly with Mo loading amount higher than 20 wt.%. According to the results of surface area measurements and crystallite size calculations of the as-prepared catalysts with different initial Mo loading amounts as shown in Table 1, surface area of the as-prepared catalyst decreased and crystallite size of the as-prepared catalyst increased obviously when the initial Mo loading amount was increased to 30 wt.%. This may be due to the sintering of metal particle during carburization, resulting in the decrease of surface area of the as-prepared catalyst and the catalytically active sites. This may be one of the reasons for the decrease in the gas production yield at high Mo loading amount. Another possible reason may be the existent of MoO2, which is not successfully transforming to Mo2C and has not active in steam reforming reaction [36,37], covered the active
Fig. 5 e Effect of Mo loading amount on gas production yields (Reaction temperature ¼ 650 C).
Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068
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sites of Mo2C, also leading to the decrease of the catalytically active sites. In the case of initial Mo loading amount of 20 wt.%, the highest H2 yield was obtained and about 5 times higher than that of non-catalytic system. Fig. 6 shows the effect of weight ratio of catalyst to biomass on the gas production yields from catalytic steam gasification of biomass. Here, the experiments were performed at a reaction temperature of 650 C with a water flow rate of 0.09 g/min at room temperature. The height of catalyst bed was kept constant at 8 cm by mixing the silica with catalyst. The results revealed that the catalyst to biomass ratio had a great influence on the gas production yields, especially for H2 yield. With the increase in the weight ratio of catalyst to biomass, the H2 yield increased obviously when compared to the yield of other gases. Moreover, the CO yield decreased greatly at 3.3 weight ratio of catalyst to biomass. This may be due to the strong influence from the water-gas shift reaction. As reported in many literatures [27,38,39], Mo2C is also highly active for this reaction. Therefore, in order to obtain the H2-rich gas product, the weight ratio of catalyst to biomass should be higher than 3.3 in this study. Generally, the product gas compositions and carbon conversion in the gasification process strongly depend on the reaction temperature [40]. In the present study, in order to evaluate the effect of reaction temperature on the catalytic activity of Mo2C/BC catalyst at the initial Mo loading amount of 20 wt.% in steam reforming of tar, the reaction temperature was varied from 550 to 750 C but the steam flow rate remained at a constant rate of 0.09 g/min. The results are presented in Fig. 7. It is found that gas production yield increased sharply with the increasing of reaction temperature, especially for H2, and then decreased slightly when the reaction temperature was higher than 650 C. On the one hand, higher temperature can enhance both steam gasification and the Boudouard reactions by providing the energy for endothermic gasification reactions, resulting in more volatiles and gas release. Moreover, high temperature also favors cracking and reforming of tar leading to an increase in the gas production yield [41e43]. On the other hand, at high
Fig. 7 e Effect of reaction temperature on gas production yield.
temperature, Mo2C might be oxidized to MoO2 and/or MoO3, which is not active in steam reforming reaction, by steam and produce CO2 via the following reactions, respectively [36]:
Mo2C þ 5H2O 4 2MoO2 þ CO þ 5H2
(4)
Mo2C þ 8H2O 4 2MoO3 þ CO2 þ 8H2
(5)
Mo2C þ 5CO2 4 2MoO2 þ 6CO
(6)
Darujati et al. [36] also reported that the Mo2C was oxidized to MoO2 at temperature about 600 C when the carbide was exposed to either steam or CO2. This may be the reasons for the decrease in the gas production yield at high temperature. Fig. 8 shows the XRD patterns of the as-prepared catalysts with initial Mo loading content of 20 wt.% before and after the reaction at a temperature of 650 C. As can be seen, after the reaction, a peak corresponding to MoO2 appeared and the intensity of the carbide peak decreased obviously, indicating that some Mo2C was oxidized to MoO2 during the reaction, leading to decreasing of catalytic activity. Based on these results, the optimum reaction temperature of Mo2C/BC catalyst for steam reforming of tar in this experiment was approximately 650 C at which the highest gas production yield was obtained. Table 2 shows the heavy tar amount collected from the experiments and the calculated tar amount by carbon balance in non-catalytic and catalytic steam gasification of cedar. Herein, the tar reduction is defined as: % tar reduction ¼ ðA=BÞ 100
Fig. 6 e Effect of the catalyst to biomass ratio on the gas production yields (Reaction temperature ¼ 650 C).
(7)
where A (g) is the weight difference between heavy tar collected in a non-catalytic and a catalytic system while B (g) is the weight of heavy tar collected in a non-catalytic system.
Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068
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Fig. 8 e XRD patterns of Mo2C/BC catalysts with Mo loading content of 20 wt.% (Reaction temperature ¼ 650 C).
According to the heavy tar amount, it is found that the asprepared Mo2C/BC catalyst showed a potential to remove the heavy tar in biomass gasification and the % tar reduction was found to be 99.0%. However, comparing with the calculated tar amount, the % tar reduction was found to be only 19.1%. This may be due to some of tar or light tar is difficult to be collected. Therefore, it can be assumed that the as-prepared Mo2C/BC catalyst has a good efficiency for the removal of heavy tar, which can cause many problems more easily than light tar, in biomass steam gasification process under the conditions selected in this work.
Stability and reusability of Mo2C/BC catalyst Fig. 9 shows gas production yields when the as-prepared catalysts with initial Mo loading content of 20 wt.% was used again with and without regeneration for the steam reforming of biomass tar. In the case of without regeneration, the gas production yields decreased slightly due to parts of Mo2C were oxidized to MoO2 by steam during the reaction, but MoO2 has no active for the steam reforming reaction [36,37]. On the other hand, the carbon deposition on the catalyst surface due to the cracking of tar molecules could also decrease the catalytic activity of the catalyst. Therefore, in
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Fig. 9 e Stability and reusability of Mo2C/BC catalysts with Mo loading content of 20 wt.% for steam reforming of biomass tar (Reaction temperature ¼ 650 C).
order to remain its catalytic activity, the spent catalyst should be regenerated before reuse again. In this study, in order to regenerate the deactivated Mo2C/BC catalyst, a try was performed to re-carburize it in Ar atmosphere at different temperatures and the results are shown in Fig. 8. From the XRD patterns, it can be seen that pure Mo2C phase can be only obtained when the spent catalyst was re-carburized at a temperature above 900 C for 2 h in Ar atmosphere. As mentioned above in Section 3.1.1, in the case of when using cedar char as a carbon source, the carburization temperature should be also increased above 900 C in order to obtain pure Mo2C supported on char. Therefore, the spent catalyst was regenerated by re-carburized at a temperature above 900 C for 2 h in Ar atmosphere and its catalytic activity is shown in Fig. 9. It is found that the gas production yield exhibited a little bit higher than the original one, indicating that the catalytic activity of the spent catalyst can be recovered when MoO2 was re-carburized into Mo2C. In addition, a part of alkali and alkaline earth metallic species could be moved from the biomass with tar and deposited on the catalyst, resulting in its activity promoted to some extent [44]. From these results, it is suggested that the as-prepared catalysts exhibited good catalytic activity and reusability.
Conclusions Table 2 e Tar amount from steam gasification of biomass at reaction temperature of 650 C. System
Non-catalytic Catalytica a
Heavy tar amount (g for 0.6 g biomass)
Calculated tar amount by carbon balance (g for 0.6 g biomass)
0.1715 0.0017
0.1778 0.1438
Mo loading amount ¼ 20 wt.%.
The findings of the present study can be summarized as follows: 1) Mo2C/BC catalyst was successfully prepared by in-situ solid state reaction using original biomass as the carbon source. Pure Mo2C phase was achieved at relatively low carburization temperature of 800 C and the initial Mo loading amount lower than 30 wt.%. 2) The as-prepared Mo2C/BC catalyst exhibited a good catalytic activity for the steam reforming of heavy tar to
Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068
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produce H2 and easily regenerated. In the case of 20 wt.% of initial Mo loading amount, the highest H2 yield was obtained and about 5 times higher than that of non-catalytic system. 3) Reaction temperature had a significant effect on the gas production yield. The optimum reaction temperature of Mo2C/BC catalyst for steam reforming of tar in this experiment was found to be 650 C.
Acknowledgments This work is supported by Japan Science and Technology Agency (JST), Strategic International Collaborative Research Program (SICORP), Japan and Aomori City Government. M. Kaewpanha gratefully acknowledges the scholarship from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and research fund for PhD course student from Hirosaki University. Mr Ma thanks the State Scholarship Fund of China Scholarship Council (2012). The authors also thank Dr Yutaka Kasai and Dr Seiji Kakuta at Aomori Prefectural Industrial Technology Research Center for their technical support on experiments.
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Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst Malinee Kaewpanha a, Guoqing Guan a,b,*, Yufei Ma a, Xiaogang Hao c, Zhonglin Zhang c, Prasert Reubroychareon d, Katsuki Kusakabe e, Abuliti Abudula a,b,* a
Graduate School of Science and Technology, Hirosaki University, 1-Bunkyocho, Hirosaki 036-8560, Japan North Japan Research Institute for Sustainable Energy (NJRISE), Hirosaki University, 2-1-3, Matsubara, Aomori 030-0813, Japan c Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China d Department of Chemical Technology, Chulalongkorn University, Bangkok 10330, Thailand e Department of Nanoscience, Sojo University, 4-22-1 Ikeda, Nishi-ku, Kumamoto 860-0082, Japan b
article info
abstract
Article history:
Catalytic steam reforming of tar derived from Japanese cedar pyrolysis was investigated
Received 2 March 2015
over a prepared biomass char supported molybdenum carbide (Mo2C/BC) catalyst in a
Received in revised form
fixed-bed reactor. Mo2C/BC was prepared by in-situ solid state reaction, and characterized
26 March 2015
by XRD and SEM to study the effects of carburization temperature and Mo loading amount
Accepted 14 April 2015
on Mo2C formation. The results show that Mo2C/BC catalyst was successfully prepared at a
Available online xxx
relatively low carburization temperature of 800 C and the initial Mo loading amount for the preparation of this catalyst should be lower than 30 wt.%. The Mo2C/BC showed a good
Keywords:
catalytic activity for the steam reforming of tar derived from biomass. In the case of
Biomass
Mo2C/BC with a Mo loading of 20 wt.%, the highest H2 yield was obtained, which is about 5
Tar reforming
times higher than that of non-catalytic test. Mo2C/BC could be a promising catalyst in
Molybdenum carbide
steam gasification of biomass to remove tar and produce H2-rich gas.
Biomass char
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Catalytic activity
Introduction Nowadays, energy and environmental issues are two common concerns that should be managed simultaneously. At present, although fossil fuels such as oil, coal and natural gas are the
reserved.
major energy sources for global energy requirement, they are finite and will be depleted eventually, and also the major cause for global warming due to the emission of greenhouse gas during combustion of fossil fuels. According to the two issues stated above, considerable efforts have been made to develop or find the alternative fuels that can solve both the
* Corresponding authors. North Japan Research Institute for Sustainable Energy (NJRISE), Hirosaki University, 2-1-3, Matsubara, Aomori 030-0813, Japan. Tel.: þ81 17 762 7756; fax: þ81 17 735 5411. E-mail addresses: [email protected] (G. Guan), [email protected] (A. Abudula). http://dx.doi.org/10.1016/j.ijhydene.2015.04.068 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068
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mentioned problems. Hydrogen has been proposed as one of the most promising energy carriers in the future due to its cleanliness and high-energy yield. However, approximately 96% of hydrogen is produced from fossil fuels, mainly by steam reforming of natural gas. In such cases, a significant amount of CO2 is released during the hydrogen production [1,2]. Biomass gasification is a promising way to produce hydrogen or synthesis gas, which is a high value gaseous product and can be applied in fuel cells for power generation or production of liquid fuels [3e7]. Biomass is renewable, abundant and carbon neutral. Nevertheless, the major problem of biomass gasification is the formation of tar, which is a complex mixture of aromatics and undesirable products. Tar can remain liquid at room temperature, causing plugging, fouling and corrosion in the equipment downstream [8e12]. Therefore, tar removal step from the syngas is imperative before syngas applications. There are several methods to reduce the tar content in raw fuel gas by physical and chemical processes. Among them, catalytic steam reforming is a very attractive technique for tar removal since it can remove tar effectively and simultaneously convert tar into useful gaseous products (H2 and CO) [13e16]. Many kinds of catalysts have been developed for catalytic conversion of tar. Noble metal catalysts such as Ru, Rh, and Pt showed high catalytic activity in the steam reforming of tar with high sulfur resistance and long term stability [17,18]. However, noble metals are expensive, making them not be used in the conventional process. Ni-based catalysts have been used extensively for biomass tar conversion because of their high tar destruction activity. However, the main limitation of nickel catalysts is the rapid deactivation, caused by carbon formation on the catalyst surface [19e21]. Moreover, other metal catalysts like Co, Fe, Zn, and Cu have also been investigated in steam reforming of tar and showed higher catalytic activity than Ni catalyst in some cases. Although other metal catalysts exhibit a good performance in steam reforming of tar, they are still deactivated easily by sulfur or high heavy tar content [1,22e24]. In recent years, molybdenum carbide (Mo2C) has been reported to have high catalytic activity similar to precious metals in various reactions such as methanol reforming, hydrogenation reactions, water gas shift, and methane reforming [24e28]. Especially, the raw materials for synthesis of Mo2C are abundant and inexpensive. Generally, metal carbide catalysts could be prepared through pyrolysis, self-propagating high temperature synthesis and direct carburization at high temperature up to 2000 C. However, these methods require very specialized high temperature equipment, gadget and facilities [29]. In order to reduce the production cost of such catalysts, several methods have been developed for the preparation of metal carbide at relatively low temperature. For instance, Mordenti et al. [30] have reported that the formation of Mo2C phase at low temperature (1000 C) was obtained by reaction between the transition metal oxide and high specific surface activated carbon. Chen et al. [31] studied the synthesis of Mo2C which is covalently anchored to carbon supports, and found that active catalysts (Mo2C) can be produced by solidestate reaction of original biomass (soybean) and ammonium molybdate at relatively low temperature (800 C). In this work, we tried to use carbon in original biomass as the carbon source
and find a simple preparation method to prepare Mo2C based catalysts. Herein, Mo2C/cedar char catalysts were prepared by in-situ solid state reaction. The obtained catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and BET surface area measurements, and then tested their catalytic activities for the steam reforming of tar derived from biomass pyrolysis. The effects of Mo loading amount and carburization temperature on the formation of molybdenum carbide and their catalytic activities were investigated and discussed.
Experimental Materials and catalyst preparation In this study, cedar wood (with particle size of 1.0e2.8 mm) was used as the carbon sources. It was dried in an oven at 105 C before storage and further use. The water content of the dried cedar wood was 8.0 wt.%. The water-free compositions by weight percentage were C 48.8%, H 6.6%, O 43.0%, N 1.4%, and ash 0.6%. From XRF analysis, the main compositions in ash by weight percentage were CaO 50.68%, SO3 10.47%, SiO2 6.83%, P2O5 6.45%, K2O 4.01%, Fe2O3 0.80% and SrO 0.04%. Ammonium heptamolybdate ((NH4)6Mo7O24$4H2O) was purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan) and used directly. Biomass char supported Mo2C (Mo2C/BC) catalyst was prepared by the following procedures: (1) dried cedar was directly impregnated by wet impregnation method with an aqueous solution of ammonium heptamolybdate of appropriate concentration to load 5-30 wt.% on initial Mo basis; (2) the slurry was dried at 110 C overnight; (3) the solid mixture was carburized in a vertical tube furnace with a 50 cm3/min Ar flow from room temperature (RT) to the desired temperatures and then held for 2 h. The carburization was performed at the temperature range of 600e800 C. Ar was kept purging through the reactor when the reactor was cooled down to room temperature before storage and further use. To evaluate the effect of carburization temperature on the Mo2C formation, ammonium heptamolybdate deposited dried cedar (initial Mo loading amount of 10 wt.%) was carburized at different carburization temperatures of 600, 700 and 800 C, separately. The samples were heated at rapid rate from room temperature to the final carburization temperature and held at that temperature for 2 h. For regeneration of the asprepared Mo2C/BC catalyst, the spent catalyst was recarburized in Ar atmosphere at temperature of 800 and 900 C, separately, using the same procedure as a carburization of Mo2C/BC catalyst.
Characterization of catalyst XRD analysis was conducted using a XRD 610 (Shimadzu, Japan) to determine the crystal structure of the as-prepared catalyst. The morphology of as-prepared catalyst was characterized using a SEM (SU6600, Hitachi) equipped with energy dispersive spectrometer (EDS). Catalyst surface areas and pore volumes were measured using BET sorption isotherm method (Quantachrome NOVA 4200e, USA)
Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068
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Catalytic activity test Catalytic activity in the steam reforming of tar was tested in a down-flow fixed-bed reactor with an internal diameter and a length of 18 and 350 mm, respectively. The experimental setup was described in details in our previous study [32]. 0.6 g of cedar powders and Mo2C/BC catalyst with different amounts were introduced into the reactor and separated with a layer of quartz wool. The height of catalyst bed was kept constant about 8 cm for all experiments. The reactions were carried out at atmospheric pressure. Reaction temperatures were chosen in the range of 550e750 C. Heating rate was fixed at 20 C/min. Water with a flow rate of 0.09 g/min was introduced using a peristaltic pump into a steam generator (250 C) at first, and then was carried to the reactor by argon gas with a flow rate 50 cm3/min. The produced gases were passed through two cooling baths under 0 C and a dry cylinder with CaCl2 particles, and collected using a gas bag. The gas compositions were analyzed using an Agilent 7890A gas chromatograph system. The carbon content in the remained char was estimated by using Vario EL cube elemental analyzer. The heavy tars were collected in the bottle with acetone, and then the water and acetone were evaporated at 100 C for 24 h in an oven, and the remained heavy tars were weighed. For the catalytic system, the gas production yield (Ybio) from the gasification of cedar and the catalytic steam reforming of tar derived from cedar was calculated as follow: Ybio ¼ Ytotal Ycat
(1)
where Ytotal is the gas production yield in the catalytic system (consisting of the gas production yield from the gasification of cedar, the catalytic steam reforming of tar derived from cedar and the gasification of catalyst itself) while Ycat is the gas production yield when catalyst itself was gasified at the same condition.
Results and discussion Characterization of the as-prepared molybdenum carbides Effect of carburization temperature As stated above, Mo2C/BC catalysts were prepared by in-situ solid state reaction from dried cedar and ammonium heptamolybdate. As shown in Fig. 1, it can be seen that at a low carburization temperature, i.e., 600 C, only molybdenum oxide (MoO2) phase was observed. As the temperature was increased to 700 C, it is found that the signal of MoO2 phase decreased and the signal of Mo2C began to appear. When the temperature was increased to 800 C, the XRD pattern shows the peaks of pure Mo2C, indicating that Mo2C was successfully prepared by in-situ solid state reaction of dried cedar and ammonium heptamolybdate. All these results indicate that the minimum carburization temperature for Mo2C formation is 800 C under the conditions selected in this work. According to the XRD results, as the Mo2C formation pathway reported in literatures [33,34], the carburization steps for Mo2C/BC catalyst were illustrated in Scheme 1.
Fig. 1 e XRD patterns of carburized products at different carburization temperatures.
Ammonium heptamolybdate should be decomposed to MoO3 (Mo6þ) at the lower temperature step and then, the generated MoO3 (Mo6þ) is continuously reduced to MoO2 (Mo4þ). Thereafter, MoO2 (Mo4þ) is continuously reduced to metallic molybdenum and further transformed into Mo2C at the high carburization step. Especially, the use of dried biomass instead of pure carbon materials (such as biomass char) as the carbon source in this study could promote the Mo2C formation and reduce the carburization temperature. This is due to the reducing gases such as CO and H2, which are produced during biomass pyrolysis. These can provide reduction atmosphere to reduce the molybdenum oxide, leading to the formation of Mo2C more easily. Furthermore, carbon monoxide and methane are considered as reduction and carburizing gases, respectively, for metal carbide preparation [30]. For comparison, we had ever used cedar char as the carbon source, and found that the carburization temperature should be increased to over 900 C in order to obtain pure Mo2C supported on char (as shown in Fig. 2).
Effect of Mo loading amount In order to investigate the effect of Mo loading amount on the Mo2C formation, dried cedar was impregnated with an aqueous solution of ammonium heptamolybdate with different contents in the range of 5e30 wt.% on Mo basis. The samples were carburized at a temperature of 800 C for 2 h and characterized by XRD. Fig. 3 shows the XRD patterns of carburized products prepared under different Mo loading amounts. It is found that pure Mo2C phase can be obtained at initial Mo loading amount below 30 wt.%, and the signals of Mo2C phase increased with the increase in Mo amount due to the formation of better-crystalline Mo2C particles. However, further increased initial Mo loading amount to 30 wt.%, the XRD pattern showed two different crystalline phases (1) a main Mo2C phase, and (2) traces of MoO2 phase. It is indicated that due to the excess of Mo loading amount in this
Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068
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Scheme 1 e Proposed formation process of biomass char supported Mo2C catalyst by in-situ solid state reaction.
experiment, some Mo could still remain in the oxidation state. pez et al. [34], Similar results were also reported by Guil-Lo who investigated the influence of the Mo-loading on the crystalline Mo-phases during the carburization processes. Their results revealed that at high Mo-loading amount, the carburization of the molybdate precursor was not complete and a higher carburization temperature was required to fulfill Mo-carbide formation. Therefore, based on the XRD results, the initial Mo loading amount for Mo2C/BC catalyst in this study should be lower than 30 wt.% at which well-crystalline Mo2C was generated in this one-step solidesolid carburization process at relatively low carburization temperature of 800 C. The BET surface area and pore volume of the as-prepared Mo2C/BC catalyst with different initial Mo loading amount are given in Table 1. It can be seen that the surface area and pore volume of the samples decreased with increasing of Mo loading amount. However, all catalysts have high surface areas, which
Three main ways which can enhance the biomass gasification rate and increase the hydrogen production yield are always considered. That is, (1) using steam gasification instead of partial oxidation, (2) increasing reaction temperature, and (3) catalyst assistance [35]. In the present study, the catalytic activity tests of the as-prepared Mo2C/BC catalysts toward tar steam reforming were performed in a
Fig. 2 e XRD patterns of carburized products at different carburization temperatures when using cedar char as carbon source.
Fig. 3 e XRD patterns of carburized products at different Mo loading amounts (Carburization reaction temperature ¼ 800 C).
are benefit for the catalyst. Scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) was also used to explore the morphology of the Mo2C/BC catalysts. Fig. 4 shows the SEM images and the EDS mapping image of ammonium heptamolybdate deposited dried cedar (initial Mo loading amount of 20 wt.%) after carburization at 800 C for 2 h. It indicated the existence of Mo with good dispersion on the surface of the as-prepared catalyst.
Catalytic activity of Mo2C/BC catalyst
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Table 1 e Surface area, pore volume and crystallite size of the as-prepared catalysts with different initial Mo loading amounts. Initial Mo loading amount (wt.%) 5 10 15 20 30 a
Surface area (m2 g1)
Pore volume (cm3 g1)
Crystallite sizea (nm)
422.0 362.3 323.4 317.8 247.1
0.231 0.201 0.198 0.193 0.169
24.2 24.6 25.4 25.7 29.9
Crystallite size of Mo2C is estimated using the Scherrer formula.
fixed-bed reactor with different conditions. In general, when tar cracking is performed in the presence of steam, the general steam reforming reaction can be expressed as following:
Fig. 4 e SEM (a,b) and EDS mapping (c) photographs of Mo2C/cedar char catalyst (Mo loading amount ¼ 20 wt.%; carburization reaction temperature ¼ 800 C).
Cx Hy Oz þ ðx zÞH2 O/xCO þ ½ðx þ y=2 zÞH2
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(2)
and the produced CO could react further with excess steam and is converted to CO2 and H2 by water-gas shift (WGS) reaction:
CO þ H2O 4 CO2 þ H2
(3)
Fig. 5 shows the yield (Ybio) of each gas, which was calculated by Eq. (1), from catalytic steam gasification of biomass using Mo2C/BC catalysts with different initial Mo loading amounts. Here, the experiments were performed at a reaction temperature of 650 C with a water flow rate of 0.09 g/min at room temperature. Results were compared to those obtained from non-catalytic system. It is found that significant changes in the gas production yields due to the presence of catalyst, especially for H2 and CO2. According to Eqs. (2) and (3), the results can be explained by the fact that biomass tar is decomposed first over the catalyst to gaseous components such as CO and H2 and then, the produced CO reacts further with excess steam by water-gas shift reaction, leading to high H2 and CO2 yields. For different initial Mo loading amounts, the results showed that Ybio increased with increasing of Mo loading amount at the beginning, and decreased slightly with Mo loading amount higher than 20 wt.%. According to the results of surface area measurements and crystallite size calculations of the as-prepared catalysts with different initial Mo loading amounts as shown in Table 1, surface area of the as-prepared catalyst decreased and crystallite size of the as-prepared catalyst increased obviously when the initial Mo loading amount was increased to 30 wt.%. This may be due to the sintering of metal particle during carburization, resulting in the decrease of surface area of the as-prepared catalyst and the catalytically active sites. This may be one of the reasons for the decrease in the gas production yield at high Mo loading amount. Another possible reason may be the existent of MoO2, which is not successfully transforming to Mo2C and has not active in steam reforming reaction [36,37], covered the active
Fig. 5 e Effect of Mo loading amount on gas production yields (Reaction temperature ¼ 650 C).
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sites of Mo2C, also leading to the decrease of the catalytically active sites. In the case of initial Mo loading amount of 20 wt.%, the highest H2 yield was obtained and about 5 times higher than that of non-catalytic system. Fig. 6 shows the effect of weight ratio of catalyst to biomass on the gas production yields from catalytic steam gasification of biomass. Here, the experiments were performed at a reaction temperature of 650 C with a water flow rate of 0.09 g/min at room temperature. The height of catalyst bed was kept constant at 8 cm by mixing the silica with catalyst. The results revealed that the catalyst to biomass ratio had a great influence on the gas production yields, especially for H2 yield. With the increase in the weight ratio of catalyst to biomass, the H2 yield increased obviously when compared to the yield of other gases. Moreover, the CO yield decreased greatly at 3.3 weight ratio of catalyst to biomass. This may be due to the strong influence from the water-gas shift reaction. As reported in many literatures [27,38,39], Mo2C is also highly active for this reaction. Therefore, in order to obtain the H2-rich gas product, the weight ratio of catalyst to biomass should be higher than 3.3 in this study. Generally, the product gas compositions and carbon conversion in the gasification process strongly depend on the reaction temperature [40]. In the present study, in order to evaluate the effect of reaction temperature on the catalytic activity of Mo2C/BC catalyst at the initial Mo loading amount of 20 wt.% in steam reforming of tar, the reaction temperature was varied from 550 to 750 C but the steam flow rate remained at a constant rate of 0.09 g/min. The results are presented in Fig. 7. It is found that gas production yield increased sharply with the increasing of reaction temperature, especially for H2, and then decreased slightly when the reaction temperature was higher than 650 C. On the one hand, higher temperature can enhance both steam gasification and the Boudouard reactions by providing the energy for endothermic gasification reactions, resulting in more volatiles and gas release. Moreover, high temperature also favors cracking and reforming of tar leading to an increase in the gas production yield [41e43]. On the other hand, at high
Fig. 7 e Effect of reaction temperature on gas production yield.
temperature, Mo2C might be oxidized to MoO2 and/or MoO3, which is not active in steam reforming reaction, by steam and produce CO2 via the following reactions, respectively [36]:
Mo2C þ 5H2O 4 2MoO2 þ CO þ 5H2
(4)
Mo2C þ 8H2O 4 2MoO3 þ CO2 þ 8H2
(5)
Mo2C þ 5CO2 4 2MoO2 þ 6CO
(6)
Darujati et al. [36] also reported that the Mo2C was oxidized to MoO2 at temperature about 600 C when the carbide was exposed to either steam or CO2. This may be the reasons for the decrease in the gas production yield at high temperature. Fig. 8 shows the XRD patterns of the as-prepared catalysts with initial Mo loading content of 20 wt.% before and after the reaction at a temperature of 650 C. As can be seen, after the reaction, a peak corresponding to MoO2 appeared and the intensity of the carbide peak decreased obviously, indicating that some Mo2C was oxidized to MoO2 during the reaction, leading to decreasing of catalytic activity. Based on these results, the optimum reaction temperature of Mo2C/BC catalyst for steam reforming of tar in this experiment was approximately 650 C at which the highest gas production yield was obtained. Table 2 shows the heavy tar amount collected from the experiments and the calculated tar amount by carbon balance in non-catalytic and catalytic steam gasification of cedar. Herein, the tar reduction is defined as: % tar reduction ¼ ðA=BÞ 100
Fig. 6 e Effect of the catalyst to biomass ratio on the gas production yields (Reaction temperature ¼ 650 C).
(7)
where A (g) is the weight difference between heavy tar collected in a non-catalytic and a catalytic system while B (g) is the weight of heavy tar collected in a non-catalytic system.
Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e9
Fig. 8 e XRD patterns of Mo2C/BC catalysts with Mo loading content of 20 wt.% (Reaction temperature ¼ 650 C).
According to the heavy tar amount, it is found that the asprepared Mo2C/BC catalyst showed a potential to remove the heavy tar in biomass gasification and the % tar reduction was found to be 99.0%. However, comparing with the calculated tar amount, the % tar reduction was found to be only 19.1%. This may be due to some of tar or light tar is difficult to be collected. Therefore, it can be assumed that the as-prepared Mo2C/BC catalyst has a good efficiency for the removal of heavy tar, which can cause many problems more easily than light tar, in biomass steam gasification process under the conditions selected in this work.
Stability and reusability of Mo2C/BC catalyst Fig. 9 shows gas production yields when the as-prepared catalysts with initial Mo loading content of 20 wt.% was used again with and without regeneration for the steam reforming of biomass tar. In the case of without regeneration, the gas production yields decreased slightly due to parts of Mo2C were oxidized to MoO2 by steam during the reaction, but MoO2 has no active for the steam reforming reaction [36,37]. On the other hand, the carbon deposition on the catalyst surface due to the cracking of tar molecules could also decrease the catalytic activity of the catalyst. Therefore, in
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Fig. 9 e Stability and reusability of Mo2C/BC catalysts with Mo loading content of 20 wt.% for steam reforming of biomass tar (Reaction temperature ¼ 650 C).
order to remain its catalytic activity, the spent catalyst should be regenerated before reuse again. In this study, in order to regenerate the deactivated Mo2C/BC catalyst, a try was performed to re-carburize it in Ar atmosphere at different temperatures and the results are shown in Fig. 8. From the XRD patterns, it can be seen that pure Mo2C phase can be only obtained when the spent catalyst was re-carburized at a temperature above 900 C for 2 h in Ar atmosphere. As mentioned above in Section 3.1.1, in the case of when using cedar char as a carbon source, the carburization temperature should be also increased above 900 C in order to obtain pure Mo2C supported on char. Therefore, the spent catalyst was regenerated by re-carburized at a temperature above 900 C for 2 h in Ar atmosphere and its catalytic activity is shown in Fig. 9. It is found that the gas production yield exhibited a little bit higher than the original one, indicating that the catalytic activity of the spent catalyst can be recovered when MoO2 was re-carburized into Mo2C. In addition, a part of alkali and alkaline earth metallic species could be moved from the biomass with tar and deposited on the catalyst, resulting in its activity promoted to some extent [44]. From these results, it is suggested that the as-prepared catalysts exhibited good catalytic activity and reusability.
Conclusions Table 2 e Tar amount from steam gasification of biomass at reaction temperature of 650 C. System
Non-catalytic Catalytica a
Heavy tar amount (g for 0.6 g biomass)
Calculated tar amount by carbon balance (g for 0.6 g biomass)
0.1715 0.0017
0.1778 0.1438
Mo loading amount ¼ 20 wt.%.
The findings of the present study can be summarized as follows: 1) Mo2C/BC catalyst was successfully prepared by in-situ solid state reaction using original biomass as the carbon source. Pure Mo2C phase was achieved at relatively low carburization temperature of 800 C and the initial Mo loading amount lower than 30 wt.%. 2) The as-prepared Mo2C/BC catalyst exhibited a good catalytic activity for the steam reforming of heavy tar to
Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068
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produce H2 and easily regenerated. In the case of 20 wt.% of initial Mo loading amount, the highest H2 yield was obtained and about 5 times higher than that of non-catalytic system. 3) Reaction temperature had a significant effect on the gas production yield. The optimum reaction temperature of Mo2C/BC catalyst for steam reforming of tar in this experiment was found to be 650 C.
Acknowledgments This work is supported by Japan Science and Technology Agency (JST), Strategic International Collaborative Research Program (SICORP), Japan and Aomori City Government. M. Kaewpanha gratefully acknowledges the scholarship from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and research fund for PhD course student from Hirosaki University. Mr Ma thanks the State Scholarship Fund of China Scholarship Council (2012). The authors also thank Dr Yutaka Kasai and Dr Seiji Kakuta at Aomori Prefectural Industrial Technology Research Center for their technical support on experiments.
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Please cite this article in press as: Kaewpanha M, et al., Hydrogen production by steam reforming of biomass tar over biomass char supported molybdenum carbide catalyst, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.04.068